Usage of Silicon Crystals to Determine the Avogadro and Planck Constants

Crystals produced from silicon that is highly enriched in the
²⁸
Si isotope were used to determine the Avogadro constant by means of the X-ray crystal density (XRCD) method. The results were included in the special CODATA adjustment for fixing the values of the Avogadro and Planck constants in the revised International System of Units (SI), which is scheduled to come into force on World Metrology Day, 2019. In this work, the XRCD method is outlined, results are discussed and future developments anticipated.